It is Simple:

If a game is good it is worth paying the subscription fee. 

To be honest, in today's market with the plethora of games available the subscription model makes less and less sense.   For me, one of those who bought/subbed at the beginning I had fun for...oh, about a month.  No hard feelings about the investment, that's about average shelf life for me on new titles the last several years so expectations were met.  However, with the game being subscription based, and having already played through the 'bulk' of the content levelling two characters to the cap, I am extremely unlikely to ever look at it again.   Going F2P, whether you consider it a fail or not, gives the chance however slight that I may return to the game risk-free, and if I find the situation has improved, perhaps invest more money at some point down the line.   

I'll use LOTRO as an example - I played in the beta and for a few months in at release.  Had a blast but capped & did all the content really.  I'll add that I'm not a power-gamer, I work full time so my playtime is mostly limited to weekends, and in a few months, I was 'done'.  However, on two occasions I have gone back to LOTRO for a 'few month' stint, during which time they've gotten more money out of me each time which if not for the F2p accessibility, would have been money they'd never seen.

Should it have gone F2P to begin with?  What does going F2P achieve?  It depends on what your goal is, really.  If you want the quick investment recoup, probably not.  However if you want longevity and a consistent playerbase sooner or later you'll have to either a) pony up monthly content updates that make people feel they are getting their moneys worth and stay interested or b) allow f2p access because even if the majority of people playing never spend a cent, their presence affects the game community/economy and can make the experience better for everyone as far as groups, instancing, guilds, etc.

My personal preference to be honest is f2p or even free trial.  I'm not likely to drop £50 on a box, sight unseen, unless its something I've been following the development of for quite some time ala SWTOR.    If its for me, I have no problem paying out a monthly sub or even minitransactions for 'stuff' and to be honest, F2p Games usually get a LOT more money out of me than a sub game as I'm very likely to spend more than £15/month on extras.  

LOTROs model I feel, is good.  I don't want to sub so the only thing I miss out on is pvp which I could care less about.  Everything else is purchasable and once unlocked, permanent.  No pressure to pay a monthly sub on something, and by extension, the feeling of 'obligation to play' because I'm paying a monthly fee on it.  I unlock the content I want, skip the rest, and through gameplay you can earn the same lotro points to unlock things, which keeps the content accessible to people who may not have the financial ability to invest.

In the olden days when the market was not so saturated, I remember happily paying my monthly sub to Asheron's Call, but in those days there were epic monthly world-changing updates and it really felt that there was something 'new' from month to month.  Nowadays new content is always over some distant horizon and its much easier to unsubscribe and find something else.

I still play the game.  And still plan on subbing as well.  $15 is more than reasonable from how much I play the game.

I know people love to hate this game, especially on sites like these.  But its not that bad and I haven't had more fun in a "Themepark" MMO ever.  Its a great game to play casual, which luckily is the type of gaming I have to play now that I have morethings in my life going on.

To my point, I think that SWTOR going FTP is more of an indictment against a sub model then how good SWTOR is or isn't. 

I'm surprised that many initially subscription-based MMOs don't use Eve Online's system of a detailed free trial period to let players get their feet wet, and if they're enticed they take the dive in and buy a subscription. From what I know a lot of players of F2P-anything want to milk as much fun as they can out of a game without paying a cent, at the expense of the developers, and go by the mentality "Let some other poor sap pay for it". But nothing is truly free, and when the "milkers" as it were then demand more content without having paid anything into the game, their demands aren't really valid.

Personally I really like the full subscription with a proper trial period model. You get to test stuff without wasting your money, and if you like what you see you can pay a normally minor fee and get all the game content, and the game developer has a fully predictable income stream from the subscriptions instead of a pot-luck F2P-with-cash-shop model. I know there are some full subscription games that don't give a free trial period and I find that completely beyond moronic.